To date, all of the known activators of platelets and hemostasis are soluble and cell matrix molecules. The Kahn lab has demonstrated that interactions between podoplanin (PDPN), a transmembrane protein, and CLEC-2 receptors on the surface of platelets prevent blood-lymphatic mixing throughout life. This pathway requires platelet activation by CLEC2. Studies from the Kahn lab reveal the basis for this phenotype to be a novel form of platelet-mediated hemostasis at the lympho-venous junction that is stimulated by CLEC2 interaction with lymphatic endothelial PDPN. The Bergmeier lab has recently collaborated with the Kahn lab to demonstrate that platelet CLEC2 signaling is also required for hemostasis during vascular inflammation in the lung and skin. The proposed studies will address these new biological roles of PDPN-CLEC2 platelet activation, and investigate the mechanism by which CLEC2 signaling in platelets mediates such non-canonical hemostatic responses. Aim 1 will investigate the role of PDPN-CLEC2 signaling in preventing pulmonary and inflammatory hemorrhage, two very recently identified functions of this platelet activation pathway. Aim 2 will test whether this pathway participates in platelet responses to vessel wall injury, determine the mechanism by which the pathway prevents hemorrhage during inflammation, and identify the platelet mechanisms that protect the lymphatic network through lympho- venous hemostasis. These studies will define a recently discovered platelet activation pathway that performs hemostatic roles not previously associated with platelet activation that are highly relevant for human disease.